1. Field of the Invention
The present invention relates to gamma correction, and more specifically to a digital gamma correction circuit and a digital gamma correction method, which are mainly used in display devices such as liquid crystal displays (LCDs) and plasma display panels (PDPs).
2. Description of the Background Art
Recent years have seen an increase in use of high-definition and high image quality display devices, e.g., liquid crystal displays, in electronic equipment, such as cell phones and digital cameras, which performs digital image processing. For such display devices, a digital gamma correction circuit for performing gamma correction on inputted digital image data is indispensable for displaying an image more naturally. Also, in electronic equipment, such as scanners and printers, which performs digital image processing, the digital gamma correction circuit is indispensable for outputting more natural image data.
Conventionally, two known methods as described below have been employed in such digital gamma correction circuits used for liquid crystal displays and the like. In the first method, conversion values are suitably set depending on gamma characteristics of liquid crystal panels for which the conversion values are used, and previously stored in a look-up table (hereinafter, abbreviated as “LUT”) set in a read-only memory (ROM), and a digital gamma correction circuit reads a conversion value corresponding to the value of inputted digital image data from the LUT to perform gamma correction. In the second method, the digital gamma correction circuit subjects inputted digital image data to linear approximation in each of a plurality of predetermined divided sections, and based on the approximate straight line, the digital gamma correction circuit calculates a conversion value corresponding to the digital image data to perform gamma correction.
Some conventional digital gamma correction circuits employing the first method are configured to read correction data corresponding to a characteristic of a display device, which is previously stored in an LUT set in a read-only memory, based on data obtained by adding a predetermined offset value to an inputted video signal. Such configuration is disclosed in, for example, Japanese Laid-Open Patent Publication No. 8-51557 (Patent Document 1).
In addition, some conventional digital gamma correction circuits employing the second method are configured to divide a gamma correction curve, which is ideal for a gamma characteristic of a display device such as a liquid crystal panel, into a plurality of sections, and approximate the gamma correction curve with a plurality of straight lines connected in the vicinity of the border between two adjacent sections to obtain coefficients for primary expressions for each of the straight lines represented by the primary expressions, such that gamma correction is performed on inputted video data based on the primary expressions with the obtained coefficients. Such configuration is disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-32237 (Patent Document 2).
Further, some conventional digital gamma correction circuits employing the second method are configured to perform high-speed gamma correction on video data by approximating a gamma correction curve, which is ideal for a gamma characteristic of a display device such as a liquid crystal panel, with a plurality of straight lines, fixing the position (e.g., an X-coordinate) of a break point corresponding to inputted data and calculating the position (e.g., a Y-coordinate) of a break point corresponding to output data before receiving the video data. Such configuration is disclosed in Japanese Laid-Open Patent Publication No. 11-120344 (Patent Document 3).
However, the conventional digital gamma correction circuit disclosed in Patent Document 1 is required to have stored in the LUT the correction data corresponding to gamma characteristics of display devices, such as liquid crystal panels, as conversion values for all video data. Thus, there is a problem that as the number of bits in inputted video data increases, the number of correction data items that have to be previously stored becomes significantly large.
In addition, the conventional digital gamma correction circuits disclosed in Patent Documents 2 and 3 are required to separately set suitable division points for the above-described sections in order to deal with display devices with various gamma characteristics, and therefore in order to achieve a smooth correction characteristic similar to a gamma correction curve ideal for each display device, it is necessary to increase the number of divided sections. However, these conventional digital gamma correction circuits are required to separately set positions of break points corresponding to input data and output data in accordance with the number of divided sections, and therefore there is a problem that the number of data items concerning the positions that have to be previously set increases with the number of divided sections.